(1) Field of the Invention
The present invention relates to an idle rotation speed learning control apparatus and method of an engine, more specifically to an apparatus for controlling an intake air quantity of the engine so that the engine rotation speed when driven in an idle state becomes a target idle rotation speed, and especially to a technique for learning and correcting the portion corresponding to the decreasingly changed portion of an opening area by age of the engine intake system caused by fouling, clogging and the like.
(2) Related Art of the Invention
Conventionally, in the engines mounted in vehicles an engine intake air quantity is feedback controlled during the idle driving of the engine, so that an engine rotation speed approximates a target idle rotation speed.
Moreover, when a predetermined learning condition is fulfilled, the feedback correction quantity for gaining the target idle rotation speed is learned as a decreasingly changed portion of the opening area by age of the intake system caused by fouling and clogging.
In recent years, a lean burn engine performing the combustion by an air-fuel ratio of approximately 20 to 25, or a direct injection gasoline engine is developed which enables combustion by a super lean air-fuel ratio of approximately 40 to 50 by performing a stratified combustion where fuel is injected directly into a cylinder.
In such engines, the improvement of the fuel consumption and the exhaust emission are attempted by performing a lean burn during a low rotation or a low load region including the idle state. Therefore, when applying the conventional idle rotation speed learning control as it is to the lean burn engine or the direct injection gasoline engine, the learning will be performed in a lean burn state.
However, when combustion is performed in a lean air-fuel ratio, the request intake air quantity of the engine will be large in comparison to the case where combustion is performed in a theoretical air-fuel ratio. Therefore, the ratio of the decreasingly changed portion of the air quantity caused by fouling and clogging in the whole intake air quantity is relatively small. Therefore, in a structure where the learning of the idle rotation speed is performed in a lean burn state, there was a problem that a highly accurate learning of the decreasingly changed portion of the opening area caused by fouling and clogging is difficult.